TM4C1294NCPDT: Ethernet connection to transformer - center taps

David M. Alter

Part Number: TM4C1294NCPDT
Other Parts Discussed in Thread: TLK110

Tiva team,

The TM4C1294NCPDT datasheet shows the proper connection to an Ethernet transformer:

Specifically, on the HX1188FNL, the center taps '2' and '7' on the PHY side are connected to 3V3. The question is why is this necessary?

I tried researching this on the web. From what I could find, the center tap to Vdd (3V3) connection is made when the TX and RX signals are differential pairs (open drain pins). The 3V3 connections of the center taps are needed to bias the transformer. This is a bit too high level of an explanation though.

A customer has apparently connected the center taps to ground, and the Ethernet seems to be working fine. I think this needs to be changed, but I cannot explain electrically what having the center taps connected to 3V3 does and why it is necessary.

Thank you and regards,

David

over 5 years ago

0 Charles Tsai over 5 years ago

TI__Guru**** 176826 points

Hi Dave,

I'm not an expert on how the center tap should be connected. My understanding is that the center tap on the transformer PHY side is normally connected to ground through a capacitor, providing a low impedance path for noise. This is a bit in contrary to the diagram shown in the TM4C129 datasheet where the center tap is not only connected to the cap and ground but also to the VDD. Since the TM4C129 Ethernet PHY is based off of the TI TLK110 Ethernet PHY (see below diagram), the TLK110 datasheet also shows the similar center tap connection as in TM4C129.

I have done some searches on the web as well. Some mentioned just to connect the center tap to ground via ground only but I also came across this below diagram. I think it depends on the PHY to which you are interfacing